Enforce cargo fmt

[tmandry]

Closes #198.

[rust-highfive]

r? @nikomatsakis

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[tmandry]

Feel free to merge some things in ahead of this and I'll rebase.. it'll be easier that way.

[nikomatsakis]

Travis seems grumpy. One thing:

$ RUSTC_BOOTSTRAP=1 cargo fmt --all -- --check
error: 'cargo-fmt' is not installed for the toolchain 'nightly-x86_64-unknown-linux-gnu'
To install, run `rustup component add rustfmt`
The command "RUSTC_BOOTSTRAP=1 cargo fmt --all -- --check" exited with 1.

Other things:

error[E0432]: unresolved import `ena::unify`
 --> chalk-solve/src/infer.rs:3:5
  |
3 | use ena::unify as ena;
  |     ^^^^^^^^^^^^^^^^^ no `unify` in `unify`
error[E0432]: unresolved import `ena::unify`
 --> chalk-solve/src/infer.rs:3:5
  |
3 | use ena::unify as ena;
  |     ^^^^^^^^^^^^^^^^^ no `unify` in `unify`

although those things might just be bugs that were fixed

[bjorn3]

The leading :: got removed.

[tmandry]

Oops, my nightly was outdated. Trying again.

I also enable Travis caching of cargo in this PR. I don't anticipate any problems with this but if you know of any I'll take it out.

[bjorn3]

This will grow the cache every time a different nightly is used, because of rust-lang/cargo#6229

[nikomatsakis]

Do we care ?

[nikomatsakis]

That is, I don't really know what the implications of that are.

[bjorn3]

The cache growth will cause CI time to go up, because the whole target dir has to been downloaded, unpacked, repacked and uploaded.

[tmandry]

That is less than ideal, but if the cache grows too big, we have a button on Travis to delete all repo caches and start fresh.

My expectation is this will save some time, both for installing rustfmt and downloading/building dependencies. But we can see the difference in build times once this is merged and decide to disable it again.

[nikomatsakis]

seems good to me! I'd like to land this, but I'm not sure whether to worry about the caching issue that @bjorn3 noted. =)

To be honest, I don't entirely know what cache: cargo does.

[tmandry]

I commented on the thread; I think we can leave it in for now.